Method and apparatus for monitoring the performance of a compressor

ABSTRACT

A method of monitoring the operation of a compressor includes sensing a parameter on a housing of the compressor from a device placed on the housing, generating a representative sensor signal in response to the sensed parameter, transmitting from the device a data signal related to the representative sensor signal, and receiving the data signal at a location remote from the housing. Other methods and an apparatus for monitoring the compressor are further disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This disclosure relates generally to devices used to monitor apparatus,and more particularly to systems and related methods for monitoring theoperation of compressors used in pipeline operations, oil refineries,chemical plants, petrochemical plants and in other industries.

2. Discussion of Related Art

In the gas transmission industry, it has become necessary to operatelarge reciprocating type compressors that drive the movement of gaswithin a pipeline continuously without interruption, with the goal ofachieving one hundred percent efficiency. Engine driven compressors mayoperate at high speeds (e.g., 500-1000 rpm), and are therefore subjectto failing due to normal wear and tear. In addition, since pipelinecompressors often operate unattended for protracted periods of time inremote locations, it is imperative that compressors be monitored toensure they are operating properly.

Pipeline compressors often operate unattended for protracted periods inremote locations and may require a host of instrumentation to ensurefailsafe operation. Periodically, the efficiency of the compressors maybe measured to determine the operation of the compressor, as well as tomeasure performance characteristics of the compressor.

To measure efficiency of the compressor, it may be necessary todetermine the work performed by the compressor. Prior techniques haveinvolved the calculation of indicated horsepower through the measurementof pressure in the cylinder head or at the discharge and suction sidesof the compressor. Problems may exist with such methods, however, inthat pressure sensors may be expensive, inherently produce signalerrors, and have limited durability, thereby resulting in lost time andefficiency during replacement of failed pressure sensors. Accuratemeasurement of compressor cylinder pressure may be hampered by theacoustic distortion introduced by the measurement channel between thecylinder and the installed pressure sensor. This distortion isparticularly severe on high-speed compressors. Moreover, measurement ofcylinder pressure and the resulting calculated work may not include thefrictional costs of the piston riding on the cylinder walls. For largecompressors in the range of 2000 HP to 10,000 HP, or above, these lossescan be significant.

One approach to measuring work performed by a compressor is disclosed inU.S. Pat. No. 7,186,094, which is incorporated herein by reference forall purposes. U.S. Pat. No. 7,186,094 discloses an apparatus and methodfor monitoring key parameters of a reciprocating member of areciprocating piston compressor. Specifically, an apparatus for directlymeasuring rod strain on a compressor is disclosed in this patent. Thedata acquired may then be used to calculate power and work performed bythe compressor. U.S. Pat. No. 7,186,094 also discloses that otherparameters of the reciprocating members, such as the temperature of thecross-head bushing, may be measured.

SUMMARY OF THE INVENTION

One aspect of the disclosure is directed to a method of monitoring theoperation of a compressor. In one embodiment, the compressor has ahousing and a reciprocating member disposed within the housing. In acertain embodiment, the method comprises: sensing a parameter on thehousing from a device placed on the housing; generating a representativesensor signal in response to the sensed parameter; transmitting from thedevice a data signal related to the representative sensor signal; andreceiving the data signal at a location remote from the housing.

Embodiments of the method may include, when sensing a parameter,monitoring a load on the housing. Transmitting a signal may comprisemanipulating the representative sensor signal. Generating arepresentative signal may comprise generating a voltage signal, andtransmitting from the device a data signal may comprise manipulating thevoltage signal to a frequency signal. Sensing a parameter may comprisemounting a plurality of strain gauges on a load cell mounted on thehousing. The method may further comprise calculating a load on thehousing and/or calculating the power used by the compressor. In oneembodiment, the compressor may have a pressure inlet and a pressureoutlet, and the method further includes sensing the pressure at thepressure inlet and outlet.

Another aspect of the disclosure is directed to a method of monitoring amachine. In one embodiment, the machine has a housing and areciprocating member disposed within the housing. In a certainembodiment, the method comprises: sensing at least one parameter on thehousing of the machine from a device placed on the housing; generating arepresentative sensor signal in response to the at least one sensedparameter; transmitting from the device on the housing a data signalrelated to the representative senor signal; and receiving the datasignal at a location remote from the housing.

Embodiments of the method may include, when sensing at least oneparameter, monitoring a load on the housing. Transmitting a signal maycomprise manipulating the representative sensor signal. Sensing at leastone parameter may comprise mounting a plurality of strain gauges on aload cell mounted on the housing. The method may further comprisecalculating a load on the housing and/or calculating the power used bythe compressor. In one embodiment, the compressor may have a pressureinlet and a pressure outlet, and the method further includes sensing thepressure at the pressure inlet and outlet. Receiving the data signal mayfurther comprise manipulating the data signal. Transmitting may beperformed by a transmitter. Sensing at least one parameter may beperformed by at least one sensor.

Yet another aspect of the disclosure may be directed to a compressorcomprising a housing, a reciprocating member disposed in the housing, amotor coupled to the reciprocating member, and an apparatus. In oneembodiment, the apparatus includes a mobile assembly attachable to thehousing. The mobile assembly has a sensor, a transmitter and a powersource. The sensor is operable to measure a parameter of the housing andgenerate a representative sensor signal. The representative sensorsignal is input to the transmitter, with the transmitter being operableto transmit a data signal related to the representative sensor signal.The power source is operable to power the transmitter and sensor. Theapparatus further includes a stationary assembly having a receiveroperable to receive the data signal from the transmitter.

DESCRIPTION OF THE DRAWINGS

For a better understanding of the present disclosure, reference is madeto the drawing figures which are incorporated herein by reference and inwhich:

FIG. 1 is a schematic view of a compressor and a monitoring system ofthe disclosure; and

FIG. 2 is a partial view of a compressor and the monitoring system shownin FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

This disclosure is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The disclosure iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” or “having,”“containing,” “involving,” and variations thereof herein, is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items.

Directly measured parameters of components of a compressor are notreadily available because of the difficulties in directly accessinginternal components of the compressor and transmitting the key sensordata from the internal reciprocating member. The rapid movement andresulting forces on a monitoring sensor and associated devices makedirect monitoring very difficult and expensive when using a sensormounted on any of the internal components of the compressor. Forexample, directly measured brake horsepower may not be readily availableon compressors, such as those used in the gas pipeline industry.Typically compressors are controlled based on indirect measurements,such as torque inferred from fuel flow, suction pressure, dischargepressure, swept volume and clearance. This may lead to inaccuracies inthe measurement of the load on the engine, limits the ability to takefull advantage of the flexibility available and can result inoverloading the compressor and parts, such as the frame, crankshaft,rods and bearings. Prior methods and apparatus, such as the approachdisclosed in U.S. Pat. No. 7,186,094, teach a system for directlymeasuring parameters of a reciprocating motor or compressor port. Morespecifically, as disclosed in the U.S. Pat. No. 7,186,094 patent, anapparatus for directly measuring rod strain on a compressor is provided.The data acquired may then be used to calculate power and work performedby the compressor. Similarly, other parameters of the reciprocatingmembers, such as the temperature of the cross-head bushing may bemeasured. The resulting information can be used to control and limitpotential operating conditions leading to part overloading, reducing thelikelihood of potentially catastrophic failures.

However, although the approach disclosed in U.S. Pat. No. 7,186,094provides an improvement over prior art methods and apparatus, applyingsensors to the reciprocating member still provides design challenges.Specifically, since the reciprocating member (i.e., the piston) is aninternal component that operates at a rapid rate, a tremendous amount ofwear and tear associated with normal operation of the compressor maycompromise the reliability of such an approach. It is also much moredifficult, more expensive and less practical to install sensors oninternal compressor parts rather than on external areas of thecompressor. The methods and apparatus of the instant disclosure improveon monitoring the performance and safety of the compressor.Specifically, the methods and apparatus provide a means for measuringparameters of the reciprocating member without having to attach a deviceto the reciprocating member. Instead, a device may be attached to ahousing (sometimes referred to as an “external casing”) of thereciprocating member. In one embodiment, the device may be one or morestrain sensors. The strain sensors may be configured to measureparameters of the compressor and generate signals that are sent to acomputer or data analyzer to analyze the information to optimize theperformance and safety of the compressor.

Using the presented methods and apparatus make it possible to operatethe compressor at maximized efficiency, and in combination with othermeasurements, such as compressor temperature or enthalpy rise, fuelflow, crank shaft angle and other parameters, to locate capacity andperformance problems more accurately.

In one embodiment, the strain sensors may be comprised of full bridgestrain gauges that are mounted externally on the housing that surroundsthe piston assembly. The strain sensors may be configured to measurecompressive, tensile and bending strain on the external wall of thehousing. In a certain embodiment, data obtained by the bending straingauge may be required to analyze data obtained by the tensile andcompression strain gauges in order to calculate the work performed bythe compressor. The strain sensors may embody load cells that aremounted on a device that is located on an external surface of thehousing. These sensors may be configured to isolate strain due tobending forces thereby allowing accurate calculation of strain duesolely to axial loading. Each load cell may generate a representativestrain gauge signal that is transmitted to a computer via atransmitter/receiver system. The strain sensors, also known as loadcells, are temperature compensated in order to offset false signalsgenerated from thermal drift.

With reference to FIG. 1, a reciprocating piston compressor is generallydesignated at 10. As shown, a motor 12 is configured to supply fuel froma line 14 that is connected to the same source as the gas to becompressed by the compressor 10, although fuel could alternately beprovided by other means. The compressor 10 is constructed in the typicalmanner to include a crankshaft 16 rotated by the motor 12. Thiscrankshaft 14 may be connected to a slider 18, which is in turn may beconnected to a connecting rod 20 and a piston rod 22. As shown, thepiston rod 22 is configured to drive a piston 24. The rotation of thecrankshaft 16 causes the piston 24 to reciprocate within a cylinder 26.A typical compressor 10 may employ the shown slider 18 and connectingrod 20, but this arrangement may be modified as is known in the art. Asshown schematically in FIG. 1, a housing or external casing 28 isprovided to house and protect the components of the compressor 10. Thesecomponents are suitably coupled to the housing 28 as is known in theart.

The cylinder 26 may be provided with an inlet line 30, which isconnected to a source of gas, such as the gas line 14. An inlet valve 32may be provided to control the flow of gas from the inlet line 30 intothe cylinder 26. The cylinder 26 may also be provided with an outletline 34. An outlet valve 36 may be provided to control the flow of gasfrom the cylinder 26 to the outlet line 34. As is conventional in theart, operation of the valves 32, 36 may be controlled by pressuredifferentials so that gas is drawn from the inlet line 30 through theinlet valve 32 into the cylinder 26. Once gas enters the cylinder 26,the gas is compressed by the piston 24, with the compressed gas flowingout of the cylinder through the outlet valve 36 to the outlet line 34.

As discussed above, prior art devices, such as the apparatus disclosedin U.S. Pat. No. 7,186,094, may be used in conjunction with thecompressor to measure performance parameters of the compressor. Theapparatus may include a sensor assembly, such as a rod load monitor ortemperature sensor, an encoder, a processor, a transmitter and a powersource. The apparatus may be designed to measure one or more parametersof the reciprocating parts of the compressor, such as the crankshaftcross-head or bushing, the connecting rods, slider or piston rods, andtransmit data based on the sensed parameters to a receiver.

Contrary to the teachings of U.S. Pat. No. 7,186,094, an apparatus ofthe present disclosure, which is generally designated at 40 in FIG. 1,measures a parameter associated with a non-moving member, such as thehousing 28 of the compressor 10. Various parameters may be measured,such as strain or load, axial and transverse loading, temperature,enthalpy, or other parameters considered relevant to monitoring theperformance and efficiency of the compressor 10.

In one embodiment, the apparatus includes a mobile assembly generallyindicated at 42 and a stationary assembly generally indicated at 44. Themobile assembly 42 includes a sensor 46 that may embody a strainmonitor, preferably mounted to the housing 28 of the compressor 10, formeasuring the load or strain on the housing when the compressor is inuse. It should be understood that the strain monitor may be mounted toother non-moving parts of the compressor, such as the outer wall of thecylinder 26. The sensor 46 generates a representative sensor signal 48.When attached to the housing 28 of the compressor 10, the sensor 46 maygenerate a strain signal representative of the strain on the housing 28.In other embodiments, a temperature sensor may be employed to measurethe temperature of the housing 28. Temperature variations of the housing28 may indicate certain operating conditions of the compressor 10.

With reference to FIG. 2, in a certain embodiment, the sensor 46 mayinclude multiple strain gauges, each indicated at 50, mounted on theouter surface of the housing 28. The strain gauges 50 may be mounted onthe housing 28 to sense loads on or movement of the housing. In oneembodiment, two pairs of strain gauges 50 may be employed, with eachpair consisting of two strain gauges to measure strain in two separatedirections on the housing. As discussed above, the sensor 46 (e.g.,strain gauges 50) generates the representative sensor signal 48, whichis transmitted to a computer or data analyzer 52. In one embodiment, thesensor 46 may be wired to the computer 52 (FIG. 2) via wire lines.

In certain embodiments, other sensor assemblies may be used with thepresent apparatus to monitor other parameters of the compressor. Forexample, as discussed above, a temperature sensor (not shown) may bemounted to the housing to measure the temperature of the compressorhousing. For example, an unexpected temperature rise may indicate apotential catastrophic failure of the compressor. Monitoring such a risein temperature would allow the user to shut down the compressor forrepair prior to such a failure. In one embodiment, the temperaturesensor may be connected to the power source, for powering the sensor,and produces a temperature sensor signal which is transmitted to thecomputer and encoder, as necessary, so that the signal may betransmitted via a transmitter.

The representative sensor signal 48, such as strain gauge signal ortemperature signal, may be processed by the computer 52, which mayinclude amplifiers, filters, data storage units, a CPU, gauge bridges,AC/DC and other converters, clocks, calculators, software and otherdevices as known in the art. For example, the computer 52 may include anencoder 54 to convert analog signals to digital signals as necessary.Preferably the computer 52 may include a low noise amplifier to amplifyto a more usable level the output of the sensor signal 48. Similarly,the computer 52 may employ an electronic integrator as a negativefeedback element in the amplifier to eliminate or reduce any staticlevels in the representative signals. The representative signalstypically are output from the sensors as a voltage. As discussed below,the computer 52 may operate to generate a signal, such as a radiofrequency signal, for transmission to a stationary assembly. Thecomputer 52 may include an electronic device, such as a VCO, whichoutputs a continuous train of fixed width pulses whose frequency isproportional to the rod strain, as measured in voltage by the rod loadmonitor. Further circuitry may be used, such as for generating a staticoffset voltage to the VCO so as to establish a stable signal.

The apparatus 40 may also include a transmitter 56, which is coupled tothe computer 52. The transmitter 56 transmits a signal 58 to a receiver60 of the stationary assembly 44. The signal 58 may be an analog,digital, optical, or an RF-based signal.

Power for the operation of the various components of the mobile assembly42 of the apparatus 40 is provided with a power source 64. In oneembodiment, the power necessary to operate various electrical componentsof the apparatus 40 may be supplied through the operation of thecompressor itself. In one particular embodiment, the power source 64 maybe a battery or other source capable of being mounted to the housing 28of the compressor 10. However, a power generator, such as an inductivecoil assembly, may be preferred since the power generator wouldeliminate the need for frequent replacement.

In addition to the receiver 60 and the antenna 62, the stationaryassembly 44 may include another computer or data analyzer 66. Theantenna 62 may be configured to receive the signal 58 from thetransmitter 56 of the mobile assembly 42. The signal 58 may then bemanipulated as necessary by the computer 66, which can include decoders,clocks, pulse generators, stabilizers, a CPU, software, programs andother devices. The circuitry may be powered by the power source 64 ofthe mobile assembly 42, such as a battery, by direct wiring to anelectrical supply or other source, such as a solar power generator.

The transmitter 56 of the mobile assembly 42 and the receiver 60 of thestationary assembly 44 eliminates the need for a wire connection betweenthe two assemblies for transmission of the signals.

As shown, FIG. 2 illustrates the slider 18, connected to the crankshaftat cross-head bushing or connector (not shown). The slide 18 moveslinearly along a channel 70. A back cover 72 of the housing 28 is shownand typically a similar front cover (not shown) encloses the slider 18and piston rod 22.

The apparatus 40 of the disclosure used in conjunction with thecompressor 10 may also include pressure sensors, such as a suctionpressure sensor 74 fixed in the inlet line 30 so as to measure thesuction pressure of gas entering the compressor 10. Appropriate pressuresensors are well known and are readily available. Such a sensor 74 maygenerate a suction pressure signal 76 representative of the suctionpressure of the gas entering the compressor 10. The apparatus 40 mayfurther comprise a discharge pressure sensor 78 fixed in the outlet line34 so as to measure the pressure in this line, which is the dischargepressure of gas leaving the compressor 10. The discharge pressure sensor78 generates a discharge pressure signal 80 representative of thedischarge pressure.

The apparatus 40 is designed to reduce dependence on pressure sensorsprovided on the inlet and outlet lines 30, 34 of the compressor 10.However, the apparatus 40 may be used in conjunction with such sensors.The pressure and strain data may be used in conjunction or ascomparative data. For example, the function of the remaining sensors,such as a crank angle indicator, may be to provide the remaining datanecessary to determine the work performed by the compressor.

Other sensors, such as a fuel consumption flow rate sensor may be usedas well.

The apparatus may also include a rod location sensor 82 as is known inthe art. The rod location sensor 82 serves a similar function as thecrank angle indicator and will not be described in detail. The rodlocation sensor 82 provides a generated rod location signal 84representative of the location of the rod in its travel along itsfunction path such that the user has an indication of the location ofthe rod.

The representative signals from the stationary assembly 44, pressuresensors 74, 78, crank angle encoder and/or rod member location sensor 82are sent by respective lines to a sensor controller 86. The stationaryassembly 44 is connected to the controller 86 by line 88. The pressuresensors 74, 78 are connected to the controller 86 by line 90. The rodmember location sensor 82 is connected to the controller 86 by line 92.The sensor controller 86 is designed to receive the representativesignals from the various indicators. The controller 86 may includeamplifiers, band pass filters, data storage units, a CPU, gauge bridges,A/D converters and other devices as are known in the art. For example,the processor may convert analog signals to digital signals asnecessary. The controller 86 may include a calculator, timing and othercircuitry, converter software, storage capacity and cumulativemathematical calculations.

The sensor controller 86 may include many members and be located on oroff-site or partially off-site. That is, the controller 86 is notlimited to a single physical location. The controller 86 may compute ormonitor certain parameters on-site while transmitting these or otherparameters to an off-site control room. On-site monitoring and controlmay, for example, include emergency shut-down control in the case of anactual or impending failure. The controller 86 may be used to controlthe compressor operation. Typically, at least some of the controllerfunction is remote to the compressor site.

The sensor controller 86 may function as the central processing unitcarrying out the logic functions of the apparatus 40. The controller 86may comprise a single computer or a multiplicity of computers or othercalculator devices. The controller 86 may be located on site or remotefrom the compressor 10. It is anticipated that the controller 86 maymost likely be remote from the compressor 10 and will receive data froma plurality of compressors spread over a wide geographic area. Thecontroller 86 may contain a microprocessor, digital input and outputsubsystems, memory capacity in which is stored various mathematical andanalytical programs and software and constant data regarding thecompressor being analyzed. One of the primary functions of thecontroller 86 is to compute, using the representative data signals, thework performed by the compressor 10 during a predetermined timeinterval. The controller 86 may include the necessary formulas forrepetitive calculations of performance parameters. Preferably thecontroller 86, in conjunction with the rod load monitor, other sensorsand transmitter/receiver pair, permits continuous real-time monitoringof the compressor. Real-time and continuous work calculations can thenbe performed and monitored.

Other calculations may be made as well, such as the computation of workand power based on pressure measurements. The measurements and resultsof the calculation can then be used for optimization of the efficiencyand use of the compressor 10. That is, the resulting data from thecomputer 86 may be used to regulate the operation of the compressor 10to maximize the efficiency of the unit. Where several compressor unitsare being monitored simultaneously, the compressors can each beregulated to maximize the efficiency of the pipeline operation as awhole. The compressor utilization, health and integrity is then used bythe compressor controllers (either human or software based) to affectoperation in an optimized fashion. The optimization and regulation ofthe compressor units can be done manually, by remote transmission ordirect manipulation, or automatically through the use of computeroptimization software.

Optimization can also include automatic shut-downs where the measuredparameters indicate a failure or danger of catastrophic failure. Forexample, the temperature sensor may be an indicator of impendingfailure. A sharp temperature rise may indicate a need to turn off thecompressor.

In one embodiment, the controller may include many members and belocated on or off-site or partially off-site with respect to thecompressor. Specifically, the controller may not be limited to a singlephysical location. The controller may compute or monitor certainparameters on-site while transmitting these or other parameters to anoff-site control room. On-site monitoring and control may, for example,include emergency shut-down control in the case of an actual orimpending failure. The controller will be used to control the compressoroperation. Typically at least some of the controller function is remoteto the compressor site. The unit controller may function as the centralprocessing unit carrying out the logic functions of the device. Thecontroller may comprise a single computer or multiple computers or othercalculator devices. The controller may be located on site or remote fromthe compressor. The controller may contain a microprocessor, digitalinput and output subsystems, memory capacity in which is stored variousmathematical and analytical programs and software and constant dataregarding the compressor being analyzed. One of the primary functions ofthe controller is to compute, using the representative data signals, thework performed by the compressor during a predetermined time interval.The controller may include the necessary formulas for repetitivecalculations of performance parameters. Preferably the controller, inconjunction with the rod load monitor, other sensors andtransmitter/receiver pair, permits continuous real-time monitoring ofthe compressor. Real-time and continuous work calculations can then beperformed and monitored.

Having thus described several aspects of at least one embodiment of thisdisclosure, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe disclosure. Accordingly, the foregoing description and drawings areby way of example only.

1. A method of monitoring the operation of a compressor, the compressorhaving a housing and a reciprocating member disposed within the housing,the method comprising: sensing a parameter on the housing from a deviceplaced on the housing; generating a representative sensor signal inresponse to the sensed parameter; transmitting from the device a datasignal related to the representative sensor signal; and receiving thedata signal at a location remote from the housing.
 2. The method ofclaim 1, wherein sensing a parameter comprises monitoring a load on thehousing.
 3. The method of claim 1, wherein transmitting a signalcomprises manipulating the representative sensor signal.
 4. The methodof claim 3, wherein generating a representative signal comprisesgenerating a voltage signal, and wherein transmitting from the device adata signal comprises manipulating the voltage signal to a frequencysignal.
 5. The method of claim 1, wherein sensing a parameter comprisesmounting a plurality of strain gauges on a load cell mounted on thehousing.
 6. The method of claim 1, further comprising calculating a loadon the housing.
 7. The method of claim 1, further comprising calculatingthe power used by the compressor.
 8. The method of claim 1, wherein thecompressor has a pressure inlet and a pressure outlet, and wherein themethod further comprises sensing the pressure at the pressure inlet andoutlet.
 9. A method of monitoring a machine, the machine having ahousing and a reciprocating member disposed within the housing, themethod comprising: sensing at least one parameter on the housing of themachine from a device placed on the housing; generating a representativesensor signal in response to the at least one sensed parameter;transmitting from the device on the housing a data signal related to therepresentative senor signal; and receiving the data signal at a locationremote from the housing.
 10. The method of claim 9, wherein sensing atleast one parameter further comprises monitoring a load on the housing.11. The method of claim 9, wherein transmitting a signal furthercomprises manipulating the representative sensor signal.
 12. The methodof claim 9, wherein sensing at least one parameter comprises mounting aplurality of strain gauges on a load cell mounted on the housing of themachine.
 13. The method of claim 9, further comprising calculating aload on the housing.
 14. The method of claim 9, wherein receiving thedata signal further comprises manipulating the data signal.
 15. Themethod of claim 9, wherein the transmitting a signal is performed by atransmitter.
 16. The method of claim 9, wherein the sensing at least oneparameter is performed by at least one sensor.
 17. The method of claim9, wherein the compressor has a pressure inlet and a pressure outlet,and wherein the method further comprises sensing the pressure at thepressure inlet and outlet.
 18. A compressor comprising: a housing; areciprocating member disposed in the housing; a motor coupled to thereciprocating member; and an apparatus comprising a mobile assemblyattachable to the housing, the mobile assembly having a sensor, atransmitter and a power source, the sensor being operable to measure aparameter of the housing and generate a representative sensor signal,the representative sensor signal being input to the transmitter, thetransmitter being operable to transmit a data signal related to therepresentative sensor signal, and the power source being operable topower the transmitter and sensor, and a stationary assembly having areceiver operable to receive the data signal from the transmitter.